The invention relates to an apparatus for optically recording information on a recording medium with a beam of radiation and, in particular, to an apparatus wherein the medium undergoes an optically detectable change upon exposure to the beam so that the information can be read immediately after it is recorded.
In known optical recorders, the information is recorded by a laser beam on a recording medium, which typically is a rotating disc provided with a radiation sensitive layer. The intensity of the laser beam is modulated in accordance with the information to be recorded so that the beam forms, on the rotating disc, a series of micron-sized, spaced apart data spots which differ optically from the surrounding medium. Although various types of data registry layers have been proposed, the one most commonly used is a thin, reflective metallic film in which the data spots are pits melted by the laser beam. During recording, the beam is moved radially across the rotating disc so that the data spots are arranged in a plurality of radially spaced, circular tracks. The radial movement of the beam may be either continuous, in which case the circular tracks form turns of a continuous spiral on the disc surface, or in discrete steps, in which case the tracks are in the form of concentric circles.
In an optical recording produced in this manner, the information is encoded in the sequence of the data spots and intermediate areas by which the data spots are spaced apart in the track direction. In the case of digital data, for example, a data spot may represent a "one" and an intermediate area a "zero" or vice versa. Alternatively, the data to be recorded may be encoded in accordance with one of a number of known schemes in which pulses of different lengths are used to represent specific combinations of data bits. Such signals can then be used to modulate the laser beam so as to form data spots of several different lengths, each length representing a specific combination of data bits. One advantage of such modulating schemes is an increase in the information density of the disc.
The information recorded on the recording medium is read by scanning the tracks with a low intensity laser beam. In the case of a metallic registry layer, for example, the reflectivity of the pits is less than that of the intervening lands. The intensity of the radiation reflected from the disc is, therefore, modulated in accordance with the spatial distribution of the pits and lands along the track enabling the information stored in the pit/land pattern to be recovered upon detection of the reflected radiation. Moreover, the information can be read immediately after it is recorded on the disc. One arrangement for doing this is described in copending U.S. application, S.N. 183,504, filed on Sept. 2, 1980, now abondoned. In the system there disclosed the beam from the laser is split into two portions, one portion forming the write beam and the second portion the read beam. The read and write beams are angularly separated and projected onto the disc so that both beams are aligned in the track direction, with the read beam trailing the write beam by a small distance corresponding to a few bits. The modulated radiation reflected by the disc is then directed onto a photodiode which converts the radiation to an electrical playback signal.
As discussed in an article by K. Bulthuis et al in the August 1979 issue of IEEE Spectrum, pp. 26 to 33, such an arrangment allows the information being recorded to be monitored and drop-outs or errors caused by defects in the surface of the disc to be detected by comparing the data recovered from the disc with the original data. Upon detection of such errors, the data written on a defective sector of the disc is invalidated by recording an appropriate signal in that sector and the data is then rewritten in a new sector.
Although this type of error detection suffices to correct for errors caused by gross defects in the recording medium, it is not adequate to ensure that the data signal is mapped accurately into the spatial pit/land pattern on the disc. For example, in the case of constant angular velocity recording, the linear velocity at the disc periphery is considerably higher than the linear velocity at the disc center. Hence, if the recorded information is to be readout at a constant data rate, the length of a pit representing a given bit or combination of bits will have to be shorter at the central portion of the disc than the length of the corresponding pit at the periphery. The laser power and/or the pulse length of the modulating signal will therefore have to be adjusted in the dependence on the radial distance during recording of the information. Moreover, pit formation in the disc is a thermodynamic process which depends in a complex way on a large number of parameters such as the energy density and distribution in the write spot, the physical properties of the recording layer, exposure time and the like so that the length of the pit formed in the disc is not linearly related to the laser pulse width, i.e. the time that the laser is on. Thus at a given laser power and recording rate, for example, a 50% increase in the laser pulse width will in practice increase the length of the pit by an amount significantly larger than 50%.
These factors become increasingly more important at high data recording rates and data densities which are achieved largely by decreasing the pit size and using various modulation schemes to represent specific bit strings by pits of different lengths. The reason for this is that as the size of the pits decreases and approaches the size of the spot to which the read beam is focused onto the disc, it becomes more difficult to discriminate between pits and the intervening lands, as well as between pits of different lengths. That is, in the case of pits whose length is several times larger than the diameter of the read spot, the signal at the output of the photodiode detector will be a train of well defined rectangular pulses so that it is a relatively easy task to discriminate between pits and lands and/or to discriminate between features of different lengths by detecting the leading edges of successive pulses. However, as the pit and land lengths approach the diameter of the read spot, the pulse width approaches the rise time of the pulses and the signal will have a sinusoidal waveform. Accordingly, the points in the output signal which correspond to leading or trailing edges of a given data spot, such as a land or a pit, become less well defined and their detection involves a greater degree of error. Since the data spot detection error increases, it can be seen that more stringent requirements are imposed on the accuracy with which the input data must be mapped into the data spot pattern in order to maintain the total system errors within acceptable tolerances.